fix(ec2): ssh config forces use of supplied keys (#5636)

## Problem
Occasionally this error pops up: 

![image](https://github.com/user-attachments/assets/28630fd3-569e-4307-914d-815669686ba4)

## Solution
Add one line to ssh config that force agent to use the provided key. 
- `IdentitiesOnly yes`: only use identity file provided. 

More info here: https://www.ssh.com/academy/ssh/config

---

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License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

---------

Co-authored-by: Justin M. Keyes <[email protected]>

Author: Hweinstock

packages/core/src/shared/sshConfig.ts

@@ -170,14 +170,16 @@ export class SshConfig {
     private getBaseSSHConfig(proxyCommand: string): string {
         // "AddKeysToAgent" will automatically add keys used on the server to the local agent. If not set, then `ssh-add`
         // must be done locally. It's mostly a convenience thing; private keys are _not_ shared with the server.
-
+        // "IdentitiesOnly yes" forces agent to only use provided identity file.
+        // More details: https://www.ssh.com/academy/ssh/config
         return `
 # Created by AWS Toolkit for VSCode. https://github.com/aws/aws-toolkit-vscode
 Host ${this.configHostName}
     ForwardAgent yes
     AddKeysToAgent yes
     StrictHostKeyChecking accept-new
     ProxyCommand ${proxyCommand}
+    IdentitiesOnly yes
     `
     }
 

packages/toolkit/.changes/next-release/Bug Fix-7e333959-58a0-48ff-9368-888735328bf2.json

@@ -0,0 +1,4 @@
+{
+	"type": "Bug Fix",
+	"description": "EC2 connect: remote connection will no longer fail with 'too many authentication attempt'"
+}